It is pretty easy to do plugins and custom themes in Discourse, most look the same just because the UX is good enough and we don't live in an era of hyper customization that early 2000s forums did. A good example of a pretty customized Discourse install is the Blizzard forums: https://us.forums.blizzard.com/en/blizzard/ and Unreal: https://forums.unrealengine.com/categories?tag=unreal-engine